1. Field of the Invention
The present invention relates to a rotary-torque adjuster that adjusts rotary torque in the process of transmission of a driving force from a driver, to an ink-ribbon conveying mechanism having such a rotary-torque adjuster used for conveying an ink ribbon, and to a printer equipped with such an ink-ribbon conveying mechanism.
2. Description of the Related Art
In printers that apply ink ribbons, if an ink ribbon is loose in the process of a printing operation, the ink ribbon may wrinkle, which may lead to a print error. Generally, an ink ribbon fed from a feed spool is given a back tension in order to avoid such looseness of the ink ribbon. Moreover, in a take-up spool, a take-up force is adjusted in view of the back tension. Accordingly, these measures are taken in order to maintain a constant ribbon tension between the feed spool and the take-up spool.
There are various techniques for applying a ribbon tension to an ink ribbon. One example is providing each spool with a brake mechanism including, for example, a torque clutch or a torque limiter. In another example, a resilient member, such as a leaf spring, is used.
However, as the diameter of the ink ribbon wound around the feed spool and the diameter of the ink ribbon wound around the take-up spool change gradually in the course of printing, the torque acting on each of the spools changes. Therefore, there are various disclosed techniques for applying a constant ribbon tension to the ink ribbon.
For example, Japanese Unexamined Patent Application Publication No. 2002-234241 discloses a structure in which the rotary torque of a shaft that drives the take-up spool is determined by a sandwiching pressure of presser components. In this structure, a spring compression gear meshed with the shaft is rotated so as to change a biasing force applied to the presser components. Thus, the rotary torque is adjustable in accordance with the wound diameter of the ink ribbon around the spools.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-316139 discloses a technique in which a motor is controlled by counting the number of pulses generated in response to rotations of the take-up spool and the feed spool and then calculating a motor voltage related with motor torque and the rotating speed in accordance with the wound diameter of the ink ribbon around the spools.